[GR-19220] Fix String#scrub! (#2249)

PullRequest: truffleruby/2395

Author: eregon

spec/ruby/core/string/scrub_spec.rb

@@ -105,4 +105,18 @@
     input.scrub! { |b| "<?>" }
     input.should == "a<?>"
   end
+
+  it "maintains the state of frozen strings that are already valid" do
+    input = "a"
+    input.freeze
+    input.scrub!
+    input.frozen?.should be_true
+  end
+
+  it "preserves the instance variables of already valid strings" do
+    input = "a"
+    input.instance_variable_set(:@a, 'b')
+    input.scrub!
+    input.instance_variable_get(:@a).should == 'b'
+  end
 end

src/main/ruby/truffleruby/core/string.rb

@@ -1019,6 +1019,7 @@ def scrub(replace = nil, &block)
   end
 
   def scrub!(replace = nil, &block)
+    return self if valid_encoding?
     replace(scrub(replace, &block))
     self
   end